Objective lens driving apparatus

ABSTRACT

An objective lens driving apparatus includes a lens holder  1  that holds an objective lens  2 , and a stationary base  9  that supports the lens holder  1  by a support shaft  3  parallel to an optical axis of the objective lens  2 . A stationary yoke  11  and the magnet  8  are mounted to the stationary base  9 . A focusing coil  4  and the tracking coils  5   a  and  5   b  are mounted to the lens holder  1 . The interaction between the current in the focusing coil  4  and the magnetic field caused by the magnet  8  generates an electromagnetic force that moves the lens holder  1  along the support shaft  3 . The interaction between the current in the tracking coils  5   a  and  5   b  and the magnetic field caused by the magnet  8  generates an electromagnetic force that rotates the lens holder  1  about the support shaft  3.

This patent application is a divisional application of U.S. applicationSer. No. 10/336,751 which was filed Jan. 6, 2003, now U.S. Pat. No.6,894,850, which also claims priority based on a Japanese patentapplication No. 2002-75786 filed on Mar. 19, 2002, the contents of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an objective lens driving apparatusused in an optical data recording and reproducing apparatus or the like,for correcting a focusing error and a tracking error of a light spotformed on an optical data recording media such as an optical disk.

In the optical data recording and reproducing apparatus or the like, thedata is recorded on and/or reproduced from the optical data recodingmedia (hereinafter, referred to as a recording media) such as theoptical disk. In order to record the data on and/or reproduce the datafrom the recording media, the focus of an objective lens must beadjusted on a groove or a pit in which the data is recorded, and thelight spot must be positioned on a track. Therefore, it is necessary toprecisely control the position of the objective lens in the directionparallel to the optical axis of the objective lens and in the directiontraversing the track of the recording media. Japanese Provisional Patentpublication No. HEI 6-79383 discloses an objective lens drivingapparatus for controlling the position of the objective lens.

FIG. 16 shows the objective lens driving apparatus disclosed in theabove publication. The objective lens driving apparatus has a lensholder 101 that holds an objective lens 102. The lens holder 101 issupported by a support shaft 107 extending in parallel to the opticalaxis of the objective lens 102 so that the lens holder 101 is slidablealong the support shaft 107 and rotatable about the support shaft 107. Apair of focusing coils 104 and a pair of tracking coils 103 are fixed tothe outer surface of the lens holder 101. The focusing coils 104 aresymmetrically located with respect to the support shaft 107, and thetracking coils 103 are symmetrically located with respect to the supportshaft 107.

An outer yoke 109 and an inner yoke 105 are provided around the lensholder 101. Arc-shaped magnets 106 are fixed to the inner surface of theouter yoke 109. As shown in FIG. 17 in an enlarged scale, each magnet106 has a focusing magnet portion 106 a and a tracking magnet portion106 b. The focusing magnet portion 106 a and the tracking magnet portion106 b are divided by a groove 106 c formed at an intermediate positionof the magnet 106. The focusing magnet portion 106 a is polarized insuch a manner that the N-pole and the S-pole are arranged in thedirection parallel to the support shaft 107. The tracking magnet portion106 b is polarized in the direction perpendicular to the polarization ofthe focusing magnet portion 106 a.

Magnetic pieces 110 are secured to the outer surface of the lens holder101 in such a manner that each of the magnetic pieces 110 is alignedwith a border between the N-pole and the S-pole of the focusing magneticportion 106 a. The magnetic piece 110 is an elongated member extendingin the direction parallel to the support shaft 107.

In order to correct the focusing error, current is applied to thefocusing coils 104. The interaction between the current and the magneticfield caused by the focusing magnet portions 106 a generates anelectromagnetic force for moving the lens holder 101 in the directionparallel to the optical axis of the objective lens 102. As a result, theobjective lens 102 is moved in the direction parallel to the opticalaxis thereof, so that the focusing error is corrected. In order tocorrect the tracking error, current is applied to the tracking coils103. The interaction between the current and the magnetic field causedby the tracking magnet portions 106 b generates an electromagnetic forcefor rotating the lens holder 101 about the support shaft 107. As aresult, the objective lens 102 is moved in the direction traversing thetrack of the recording media, so that the tracking error is corrected.

As the lens holder 1 moves or rotates, a magnetic attractive force isexerted between the magnetic piece 110 and the focusing magnet portions106 a. With the magnetic attractive force, the lens holder 101 is stablyheld at a reference position in the direction parallel to the opticalaxis of the objective lens 102 and a reference position in the directionof the rotation about the support shaft 107.

However, since the above described conventional objective lens drivingapparatus uses two kinds of magnet portions respectively for focusingand for tracking, the structure (including a polarization structure) ofthe magnet becomes complicated, and the number of components increases.Therefore, the conventional objective lens driving apparatus has aproblem that the manufacturing cost increases, and the manufacturingprocess becomes complicated.

Moreover, in the above described conventional objective lens drivingapparatus, the lens holder has a backlash because of a gap between abearing portion of the lens holder 101 and the support shaft 107. Such abacklash of the lens holder 1 may result in tilt and vibration of theobjective lens 102.

SUMMARY OF THE INVENTION

This invention is intended to solve the above described problems, and anobject of the present invention is to provide an objective lens drivingapparatus capable of reducing the manufacturing cost, simplifying themanufacturing process, and restricting the tilt and vibration of theobjective lens.

According to the invention, there is provided an objective lens drivingapparatus comprising a lens holder (as a movable part) holding theobjective lens, and a stationary part. The stationary part supports thelens holder in such a manner that the lens holder is movable in thedirection of an optical axis of the objective lens and rotatable about arotation axis parallel to the optical axis. The objective lens drivingapparatus further comprises a magnet mounted to one of the lens holderand the stationary part. The magnet is polarized in the direction towardor away from the rotation axis. The objective lens driving apparatusfurther comprises a tracking coil and a focusing coil mounted to theother of the lens holder and the stationary part. The tracking coil hasa side substantially parallel to the optical axis. The focusing coil hasa side substantially perpendicular to the optical axis and to thedirection of the polarization of the magnet. The objective lens drivingapparatus further comprises a magnetic path forming member that forms amagnetic path in which a magnetic field caused by the magnet passesthrough the side of the focusing coil and the side of the tracking coil.The objective lens driving apparatus further comprises a currentsupplying member that supplies current to the focusing coil and thetracking coil. The objective lens driving apparatus further comprises anurging member that generates a magnetic force in combination with themagnet, for urging the lens holder toward a reference position in thedirection of the optical axis and in the direction of the rotation ofthe lens holder.

With such an arrangement, the objective lens can be driven in thefocusing direction and in the tracking direction by one kind of magnet,and therefore the number of components can be reduced. Additionally, themagnet having a complicated structure is eliminated, and therefore thecost of the component can be reduced. Accordingly, the manufacturingcost of the objective lens driving apparatus can be reduced, and alsothe manufacturing process can be simplified. Moreover, the backlash ofthe lens holder can be restricted by the magnetic force generated by theurging member and the magnet, with the result that tilt and vibration ofthe objective lens resulting from the backlash is restricted.

BRIEF DESCRIPTION OF THE DRAWINGS

In the attached drawings:

FIG. 1 is a front perspective view of the objective lens drivingapparatus according to Embodiment 1 of the present invention, seen fromabove;

FIG. 2 is a front perspective view of the objective lens drivingapparatus according to Embodiment 1 of the present invention, seen frombelow;

FIG. 3 is an exploded perspective view of the objective lens drivingapparatus according to Embodiment 1 of the present invention;

FIG. 4 is a front perspective view of a movable part of the objectivelens driving apparatus according to Embodiment 1 of the presentinvention, seen from below;

FIG. 5A is a schematic view showing the positional relationship betweena focusing coil and a yoke;

FIG. 5B is a schematic view showing the positional relationship betweentracking coils and the yoke;

FIG. 6 is a front perspective view of the objective lens drivingapparatus according to Embodiment 2 of the present invention, seen fromabove;

FIG. 7 is a front perspective view of a movable part of the objectivelens driving apparatus according to Embodiment 2 of the presentinvention, seen from below;

FIG. 8 is an exploded perspective view of the objective lens drivingapparatus according to Embodiment 2 of the present invention;

FIG. 9 is a rear perspective view of the objective lens drivingapparatus according to Embodiment 3 of the present invention, seen fromabove;

FIG. 10 is a front perspective view of a stationary base of theobjective lens driving apparatus according to Embodiment 3 of thepresent invention, seen from above;

FIG. 11 is a front perspective view of a stationary part including thestationary base of the objective lens driving apparatus according toEmbodiment 3 of the present invention, seen from above;

FIG. 12 is a front perspective view of the objective lens drivingapparatus according to Embodiment 4 of the present invention, seen fromabove;

FIG. 13 is a front perspective view of the objective lens drivingapparatus according to Embodiment 4 of the present invention, seen frombelow;

FIG. 14 is an exploded perspective view of the objective lens drivingapparatus according to Embodiment 4 of the present invention;

FIG. 15A is a schematic view showing the positional relationship betweena focusing coil and a yoke;

FIG. 15B is a schematic view showing the positional relationship betweentracking coils and a yoke;

FIG. 16 is a plan view of a conventional objective lens drivingapparatus; and

FIG. 17 is a perspective view of a magnet of the conventional objectivelens driving apparatus.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will be described with reference to theattached drawings.

Embodiment 1.

FIGS. 1 and 2 are front perspective views of an objective lens drivingapparatus according to Embodiment 1 of the present invention,respectively seen from above and seen from below. FIG. 3 is an explodedperspective view of the objective lens driving apparatus according toEmbodiment 1. FIG. 4 is a front perspective view of a movable part ofthe objective lens driving apparatus, seen from below. The objectivelens driving apparatus according to Embodiment 1 is mounted to anoptical disk drive device (not shown), and includes a lens holder 1 thatholds an objective lens 2 and a stationary base (a stationary part) 9that supports the lens holder 1, as shown in FIG. 1. The lens holder 1holds the objective lens 2 in such a manner that the direction of theoptical axis (z-direction) of the objective lens 2 is perpendicular tothe recording surface of the recording media. In the description below,y-direction is used to mean the direction traversing the track of therecording media in xy-plane perpendicular to z-direction. X-direction isused to mean the direction perpendicular to the y-direction in xy-plane.

In the objective lens driving apparatus according to Embodiment 1, thelens holder 1 is supported by the support shaft 3 (forming a rotationaxis) fixed to the stationary base 9. The support shaft 3 extends inparallel to the optical axis of the objective lens 2. The lens holder 1is movable along the support shaft 3, and is rotatable about the supportshaft 3. Coils are attached to the lens holder 1, for generating adriving force for moving and rotating the lens holder 1. A magnet 8 anda stationary yoke 11 are provided on the stationary base 9, to form amagnetic circuit. These components will be described in turn.

The stationary base 9 is a plate-shaped member generally elongated inone direction (x-direction). A through hole 10 is formed in thestationary base 9 and is located at a center portion in the longitudinaldirection of the stationary base 9. The through hole 10 penetrates thestationary base 9 in the direction parallel to the optical axis of theobjective lens 2. The support shaft 3 is fit in the hole 10 by means ofpress fitting, or adhesion or the like. The support shaft 3 is coatedwith a low-friction material such as fluorine resin.

As shown in FIG. 2, an opening 9 d is formed in the stationary base 9and is located at an end portion in the longitudinal direction(x-direction) of the stationary base 9. The opening 9 d allows the lightbeam to pass and to enter into the objective lens 2. Further, arectangular through hole 9 e is formed in the stationary base 9 and islocated on a part opposite to the opening 9 d with respect to thethrough hole 10. Three spherical surface portions 9 a, 9 b and 9 c areformed on the peripheral edges of the stationary base 9, each of whichforms a part of a common spherical surface. The spherical surfaceportions 9 a, 9 b and 9 c are engaged with a spherical concave formed ona member provided in the optical disk drive device, for adjusting theangle of the stationary base 9, and hence, the tilt of the support shaft3.

As shown in FIG. 3, a stationary yoke (a magnetic path forming member)11 is mounted to the stationary base 9. The stationary yoke 11 is madeof magnetic material, and includes a first wall portion 11 a and asecond wall portion 11 b extending in parallel to each other. Thestationary yoke 11 further includes a plate-shaped bottom portion 11 cconnecting the bottom ends of the first and second wall portions 11 aand 11 b. The first and second wall portions 11 a and 11 b extend inparallel to the support shaft 3, that is, in parallel to the opticalaxis of the objective lens 2. The bottom portion 11 c is fixed to thebottom of the stationary base 9 by means of screws 12 a and 12 b. In astate in which the bottom portion 11 c is fixed to the stationary base9, the inner surface of the first wall portion 11 a faces an end surface9 f of the stationary base 9 formed on an end opposite to the opening 9d, and the second wall portion 11 b penetrates the through hole 9 eupward, that is, to the lens holder 1 side, and is positioned betweenthe support shaft 3 and the first wall portion 11 a.

As shown in FIGS. 1 and 3, a plate-shaped magnet 8 is fixed to the innersurface of the first wall portion 11 a. The magnet 8 is polarized in thedirection of the thickness of the magnet 8, i.e., in the directiontoward or away from the support shaft 3. The magnet 8 faces an endsurface 1 h (that is, a fixing surface) formed on an end opposite to theobjective lens 2 of the lens holder 1.

The lens holder 1 is made of a material that is light in weight and hasa high rigidity, such as a plastic material. As shown in FIG. 4, thelens holder 1 has a plate portion 1 a generally elongated in onedirection (x-direction). A lens mounting portion 1 g is formed on theplate portion 1 a and is located at an end in the longitudinal direction(x-direction) of the plate portion 1 a. Hereinafter, the objective lens2 side (that is, the lens mounting portion 1 g side) of the lens holder1 is described as “front”, and the side opposite to the front side isdescribed as “rear”. A cylindrical portion 1 b is formed on the plateportion 1 a and is located at a center portion in the longitudinaldirection of the plate portion 1 a. The cylindrical portion 1 b isprotruded downward, i.e., toward the stationary base 9. The cylindricalportion 1 b has a bearing portion 1 e constituted by a through hole witha circular cross section.

A pair of downwardly extending wall portions 1 c are formed on both sideends of the plate portion 1 a, and extend from the center portion to therear end of the plate portion 1 a. An inclined surface 1 j is formed inthe front half of each wall portion 1 c, to which a flexible printcircuit board 7 (FIG.3) is fixed. The rear end surfaces of the wallportions 1 c constitute fixing surfaces 1 h to which the tracking coils5 a and 5 b are fixed. A rectangular portion 1 d is formed on the plateportion 1 a and is located behind the cylindrical portion 1 b. Therectangular portion id is protruded downward, that is, toward thestationary base 9. The rectangular portion 1 d includes a through hole 1f having a rectangular cross section. The above described second wallportion 11 b of the stationary yoke 11 is inserted in the through hole 1f of the rectangular portion 1 d.

A focusing coil 4 is wound around the rectangular portion 1 d of thelens holder 1 and fixed to the outer surfaces of the rectangular portion1 d. The focusing coil 4 is wound in a rectangular shape in such amanner that the focusing coil 4 has two sides extending in y-directionand two sides extending in x-direction. The focusing coil 4 thereforesurrounds the second wall portion 11 b inserted in the through hole 1 fof the rectangular portion 1 d. A part of the focusing coil 4, i.e., oneof the sides extending in y-direction, faces the magnet 8 fixed to thefirst wall portion 11 a of the stationary yoke 11.

As was mentioned above, the tracking coils 5 a and 5 b are fixed tofixing surfaces 1 h of the wall portions 1 c of the lens holder 1. Eachof the tracking coils 5 a and 5 b is wound in a rectangular shape insuch a manner that each of the tracking coils 5 a and 5 b has two sidesextending in y-direction and two sides extending in z-direction. Thetracking coils 5 a and 5 b face the magnet 8 fixed to the wall portion11 a of the stationary yoke 11.

Referring again to FIG. 3, a pair of magnetic plates 6 a and 6 b (anurging member) having rectangular openings are fixed to the rectangularportion 1 d (FIG. 4) of the lens holder 1. The magnetic plates 6 a and 6b vertically sandwich the focusing coil 4. Each of the magnetic plates 6a and 6 b is rectangular in a plane (xy-plane) perpendicular to thesupport shaft 3. The dimension of each of the magnetic plates 6 a and 6b in the direction in parallel to the support shaft 3 is sufficientlysmall relative to the magnet 8. The magnetic plates 6 a and 6 b are madeof magnetic material such as stainless steel or nickel. The magneticplate 6 a has a side that faces the magnet 8, on which two projections61 are formed. The magnetic plate 6 b has a side that faces the magnet8, on which two projections 62 are formed. The projections 61 and 62 arepositioned in hollow portions respectively surrounded by the trackingcoils 5 a and 5 b, and directly face the magnet 8.

The focusing coil 4 and the tracking coils 5 a and 5 b are electricallyconnected to a stationary part of the optical disk drive device, bymeans of the flexible print circuit board 7 (a current supplyingmember). The flexible print circuit board 7 has two end portions 7 a and7 b fixed to the inclined surfaces 1 j of the wall portions 1 c of thelens holder 1.

FIG. 5A is a schematic diagram showing a positional relationship amongthe focusing coil 4, the stationary yoke 11 and the magnet 8. As shownin FIG. 5A, the focusing coil 4 has sides 4 a and 4 c extending iny-direction and sides 4 b and 4 d extending in x-direction. Between themagnet 8 and the second wall portion 11 b of the stationary yoke 11, amagnetic field B is generated in the direction of the polarization ofthe magnet 8, that is, the direction toward or away from the supportshaft 3. The side 4 a of the focusing coil 4 is positioned in themagnetic field B. The current flowing in the side 4 a and the magneticfield B are perpendicular to each other, generating the electromagneticforce F in the direction along the support shaft 3, that is, thedirection in parallel to the optical axis of the objective lens 2.

FIG. 5B is a schematic diagram showing a positional relationship amongthe tracking coils 5 a and 5 b, the stationary yoke 11 and the magnet 8.As shown in FIG. 5B, each of the tracking coils 5 a and 5 b has sides 51and 53 extending in z-direction and sides 52 and 54 extending iny-direction. The side 51 of the tracking coil 5 a and the side 51 of thetracking coil 5 b are positioned in the magnetic field B. The currentflowing in the sides 51 and the magnetic field B are perpendicular toeach other, generating the electromagnetic force F in the direction ofthe rotation of the lens holder 1 about the support shaft 3.

The method for correcting the focusing error is described. In order tocorrect the focusing error, current is applied to the focusing coil 4.The interaction between the current and the magnetic field caused by themagnet 8 generates the electromagnetic force, which moves the lensholder 1 along the support shaft 3. By controlling the position of thelens holder 1 in the direction along the support shaft 3, the distancebetween the objective lens 2 and the recording media is adjusted, sothat the focusing error is corrected.

The movement of the lens holder 1 changes the magnetic field between themagnetic plates 6 a and 6 b and the magnet 8. As a result, a restoringforce is generated according to the displacement of the lens holder 1,and urges the lens holder 1 to a position where the lens holder 1 isstably held. In the direction of the movement of the lens holder 1, themagnetic flux density is maximum at the center of the magnet 8, so thatthe lens holder 1 is stably held when both of the magnetic plates 6 aand 6 b are the closest to the center of the magnet 8. Accordingly, whenthe current in the focusing coil 4 is stopped, the lens holder 1 ismoved to a position where both of the magnetic plates 6 a and 6 b are atequal distance from the magnet 8. This position is referred to as areference position in the direction of the support shaft 3. The shapeand thickness of the magnetic plates 6 a and 6 b are determined in sucha manner that a linear property is obtained in the range (generally,approximately within ±0.5 mm) of the correction of the focusing error ofthe objective lens 2. The linear property means a property that therestoring force is proportional to the displacement of the lens holder 1from the reference position.

Next, the method for correcting the tracking error is described. Inorder to correct the tracking error, current is applied to the trackingcoils 5 a and 5 b. The interaction between the current and the magneticfield caused by the magnet 8 generates the electromagnetic force, whichrotates the lens holder 1 about the support shaft 3. By rotating thelens holder 1, the objective lens 2 is moved in the direction traversingthe track of the recording media, so that the tracking error iscorrected.

The rotation of the lens holder 1 changes the magnetic field between themagnetic plates 6 a and 6 b and the magnet 8. As a result, a restoringforce is generated according to the amount of rotation of the lensholder 1. The magnetic plates 6 a and 6 b are stably held when thedistances from the respective projections 61 to the magnet 8 are equalto each other, and the distances from the respective projections 62 tothe magnet 8 are equal to each other. Thus, when the distances from therespective projections 61 (62) to the magnet 8 becomes unequal to eachother, the restoring force is generated to urge the lens holder 1 to aposition where the distances from the respective projections 61 (62) tothe magnet 8 are equal to each other. Accordingly, when the current inthe tracking coils 5 a and 5 b is stopped, the lens holder 1 is rotatedto a position where the distances from the respective projections 61(62) to the magnet 8 becomes equal to each other. This position isreferred to as a reference position in the direction of the rotation.The shape and thickness of the magnetic plates 6 a and 6 b aredetermined in such a manner that the linear property is obtained in therange (generally, approximately within ±0.5 mm) of the correction of thetracking error of the objective lens 2.

In addition to the restoring force described above, an attractivemagnetic force is exerted between the magnet 8 and the magnetic plates 6a and 6 b. Thus, the lens holder 1 is urged in the direction in whichthe bearing portion 1 e of the lens holder 1 abuts against the supportshaft 3. Thus, the backlash of the lens holder 1 resulting from the gapbetween the bearing portion 1 e and the support shaft 3 is restricted.Therefore, tilt and vibration resulting from the backlash of the lensholder 1 is restricted.

As described above, according to Embodiment 1, a single kind of magnetprovides the driving force of the lens holder 1 in the focusingdirection and the tracking direction. Since it is not necessary to use aplurality of magnets polarized in a plurality of directions, the numberof the components can be reduced. In addition, since it is not necessaryto use a magnet having a complicated structure, the cost of thecomponent can be reduced. Accordingly, the manufacturing cost of theobjective lens driving apparatus can be reduced, and the manufacturingprocess of the objective lens driving apparatus can be simplified.

Moreover, the attractive magnetic force exerted between the magnet 8 andthe magnetic plates 6 a and 6 b restricts the backlash of the lensholder 1 resulting from the gap between the support shaft 3 and thebearing portion 1 e of the lens holder 1. As a result, tilt andvibration of the objective lens 2 can be restricted. The magnetic plates6 a and 6 b are also used to generate a restoring force for urging thelens holder 1 to the reference position.

Additionally, the focusing coil 4 and the tracking coils 5 a and 5 b arelocated on a part opposite to the objective lens 2 with respect to thesupport shaft 3, so that the size of the focusing coil 4 can be reduced.Thus, of the total length of the focusing coil 4, the length of the partcontributing to the generation of the driving force can be increased. Asa result, the lens holder 1 can be moved and rotated with a relativelysmall electric power.

Furthermore, since the first wall portion 11 a and the second wallportion 11 b of the stationary yoke 11 are formed to sandwich thefocusing coil 4, the tracking coils 5 a and 5 b and the magnet 8therebetween, the sufficient magnetic field can be exerted on thefocusing coil 4 and the tracking coils 5 a and 5 b.

Embodiment 2.

FIG. 6 is a front perspective view of an objective lens drivingapparatus according to Embodiment 2, seen from above. FIG. 7 is a frontperspective view of a movable part of the objective lens drivingapparatus according to Embodiment 2, seen from below. FIG. 8 is anexploded perspective view of the objective lens driving apparatusaccording to Embodiment 2. In FIGS. 6 to 8, the components that are thesame as or correspond to the components shown in FIGS. 1 to 3 areassigned the same reference numerals. As shown in FIG. 6, the structureof the lens holder 1 of Embodiment 2 is different from that ofEmbodiment 1. The structures of the stationary base 9 and the stationaryyoke 11 are the same as those of Embodiment 1.

As shown in FIG. 7, the lens holder 1 has a generally elongated plateportion 1 a having a lens mounting portion 1 g formed at an end in thelongitudinal direction thereof, as in Embodiment 1. Wall portions 1 care formed on both side ends of the plate portion 1 a, as inEmbodiment 1. A rectangular portion 21 is protruded downward toward thestationary base 9 from the plate portion 1 a. The rectangular portion 21extends in the longitudinal direction of the plate portion 1 a from thecenter portion to the rear end of the plate portion 1 a. The bearingportion 1 e and the through hole 1 f described in Embodiment 1 areformed in the rectangular portion 21.

The focusing coil 22 is wound around the rectangular portion 21 andfixed to the outer surfaces of the rectangular portion 21 in such amanner that the focusing coil 22 has two sides extending in x-directionand two sides extending in y-direction. That is, the focusing coil 22surrounds the support shaft 3 (FIG. 8) inserted in the bearing portion 1e and the second wall portion 11 b (FIG. 8) inserted in the through hole1 f. Tracking coils 5 a and 5 b are fixed to the fixing surfaces 1 h,that is, the rear end surfaces of the wall portions 1 c, as inEmbodiment 1.

As shown in FIG. 8, a pair of magnetic plates 23 a and 23 b are fixed tothe rectangular portion 21 in such a manner that the magnetic plates 23a and 23 b vertically sandwich the focusing coil 22 therebetween. Themagnetic plates 23 a and 23 b are made of magnetic material such asstainless steel or nickel. The magnetic plate 23 a has a side that facesthe magnet 8, on which two projections 24 are formed. The magnetic plate23 b has a side that faces the magnet 8, on which two projections 25 areformed. The projections 24 and 25 are positioned in hollow portionsrespectively surrounded by the tracking coils 5 a and 5 b fixed to thefixing surface 1 h of the lens holder 1, and directly face the magnet 8.Other structure and operation of the objective lens driving apparatus ofEmbodiment 2 are the same as those of Embodiment 1.

As constructed above, according to Embodiment 2, the longitudinal sizeof the lens holder 1 can be reduced. Thus, the size and weight of themovable part of the objective lens driving apparatus can be reduced.Therefore, in addition to the advantages of Embodiment 1, the electricpower for operating the objective lens driving apparatus can be furtherreduced.

Embodiment 3.

FIG. 9 is a rear perspective view of an objective lens driving apparatusaccording to Embodiment 3, seen from above. FIG. 10 is a frontperspective view of a stationary base according to Embodiment 3, seenfrom above. FIG. 11 is a front perspective view of a stationary partincluding the stationary base shown in FIG. 10, seen from above. InFIGS. 9 to 11, the components that are the same as or correspond to thecomponents shown in FIGS. 1 to 3 are assigned the same referencenumerals.

In Embodiments 1 and 2 described above, the stationary base 9 and thestationary yoke 11 are separate with each other (FIG. 3). In contrast,in Embodiment 3, a stationary base 31 includes a part that functions asthe stationary yoke, as shown in FIG. 9. The stationary base 31 shown inFIG. 10 is generally elongated and is made of magnetic material. Athrough hole 32 is formed in the stationary base 31, and is located at acenter portion in the longitudinal direction of the stationary base 31.The through hole 32 penetrates the stationary base 31 in the directionparallel to the optical axis of the objective lens 2. The support shaft3 (FIG. 11) is fit in the through hole 32 by means of press fitting,adhesion or the like.

An opening 35 is formed in the stationary base 31, and is located at anend in the longitudinal direction of the stationary base 31. The opening35 allows the light beam to pass and to enter into the objective lens 2.A pair of wall portions 31 a and 31 b are formed on the stationary base31, and are located on a part opposite to the opening 35 with respectiveto the support shaft 3. The wall portions 31 a and 31 b extend inparallel to the support shaft 3. The first wall portion 31 a is locatedon an end opposite to the opening 35 with respect to the support shaft 3of the stationary base 31. The second wall portion 31 b is locatedbetween the support shaft 3 and the first wall portion 31 a. Threespherical surface portions 33 a, 33 b and 33 c are formed on theperipheral edges of the stationary base 31, each of which forms a partof a common sphere. The function of the spherical surface portions 33 a,33 b and 33 c is the same as that of the spherical surface portions 9 a,9 b and 9 c of Embodiment 1.

As shown in FIG. 11, a plate-shaped magnet 8 is fixed to the innersurface of the first wall portion 31 a of the stationary base 31. Themagnet 8 is polarized in the direction of the thickness thereof and inthe direction toward or away from the support shaft 3. As in Embodiment1, the side 4 a (FIG. 5B) of the focusing coil 4 and the sides 51 (FIG.5B) of the tracking coils 5 a and 5 b are positioned in the magneticfield caused by the magnet 8. The interaction between the current andthe magnetic field generates the driving force for moving the lensholder 1 along the support shaft 3 and rotating the lens holder 1 aboutthe support shaft 3. Other structure and operation of the objective lensdriving apparatus of Embodiment 3 are the same as those of the objectivelens driving apparatus of Embodiment 1.

As constructed above, according to Embodiment 3, the stationary base 31also functions as the stationary yoke, and therefore it is not necessaryto provide a separate stationary yoke. Accordingly, in addition to theadvantages of Embodiment 1, the manufacturing cost of the objective lensdrive apparatus can be further reduced.

Embodiment 4.

FIGS. 12 and 13 are front perspective views of an objective lens drivingapparatus, respectively seen from above and seen from below. FIG. 14 isan exploded perspective view of the objective lens driving apparatusaccording to Embodiment 4. In FIGS. 12 to 14, the components that arethe same as or correspond to the components shown in FIGS. 1 to 3 areassigned the same reference numerals.

In Embodiments 1 to 3 described above, the magnet is fixed to thestationary base directly or via the stationary yoke. In contrast, inEmbodiment 4, a magnet 41 is fixed to the lens holder 1 as shown in FIG.12.

In Embodiment 4, the lens holder 1 includes a generally elongated plateportion 1 a, as in Embodiment 1. The plate portion 1 a has a cylindricalportion 1 b formed at a center portion in the longitudinal directionthereof, and a lens mounting portion 1 g formed at an end in thelongitudinal direction thereof. The objective lens 2 side (that is, thelens mounting portion 1 g side) of the lens holder 1 is described as“front”, and the side opposite to the front side is described as “rear”,as in Embodiment 1. A rectangular through hole 1 i is formed in theplate portion 1 a, and is located behind the cylindrical portion 1 b. Inthis through hole 1 i, a second wall portion 45 b of a stationary yoke45 (described later) is inserted. A pair of wall portions 46 are formedon both sides in the width direction of the plate portion 1 a. UnlikeEmbodiment 1, the wall portions 46 do not have surfaces for fixing theflexible print circuit board. A pair of wall portions 47 extend rearwardfrom the rear ends of the wall portions 46, and supports the magnet 41.Another wall portion 48 extends in the width direction of the lensholder 1, on the front side of the magnet 41. The magnet 41 is held in aspace between the wall portions 47, in such a manner that thepolarization of the magnet 41 is in the direction toward or away fromthe support shaft 3.

The stationary yoke 45 is made of magnetic material, and includes afirst wall portion 45 a, a second wall portion 45 b and a bottom portion45 c, as shown in FIG. 14. The first wall portion 45 a and the secondwall portion 45 b face each other, and are connected by the bottomportion 45 c. The bottom portion 45 c is fixed to the stationary base 9by means of a screw 49. The first wall portion 45 a faces the magnet 41supported by the wall portions 47 at the rear end of the lens holder 1.The second wall portion 45 b is inserted in the throughhole 1 i formedin the rectangular portion 1 a of the lens holder 1.

The focusing coil 42 is wound around the upper part of the first wallportion 45 a of the stationary yoke 45 in such a manner that thefocusing coil 42 has two short sides extending in x-direction and twosides extending in y-direction. Tracking coils 43 a and 43 b are mountedon the focusing coil 4. Each of the tracking coils 43 a and 43 b iswound in a rectangular shape in such a manner that each of the trackingcoils 43 a and 43 b has two sides extending in y-direction and two sidesextending in z-direction.

FIG. 15A is a schematic view showing a positional relationship betweenthe focusing coil 42 and the stationary yoke 45. The focusing coil 42has two sides 42 a and 42 c extending in y-direction, and two sides 42 band 42 d extending in x-direction. The magnet 41 (FIG. 14) and thestationary yoke 45 form a magnetic path, so that a magnetic field isgenerated in a space between the magnet 41 and the first wall portion 45a of the stationary yoke 45. The direction of the magnetic flux of themagnetic field is the same as the direction of the polarization of themagnet 41, that is, the direction toward or away from the support shaft3. The side 42 a of the focusing coil 42 is positioned in this magneticfield. The interaction between the current in the side 42 a and themagnetic field generates the electromagnetic force in the direction ofmoving the lens holder 1 along the support shaft 3.

FIG. 15B is a schematic view showing a positional relationship betweenthe tracking coil 43 a and 43 b and the stationary yoke 45. Each of thetracking coils 43 a and 43 b has two sides 61 and 63 extending inz-direction and two sides 62 and 64 extending in y-direction. The sides61 of the tracking coils 43 a and 43 b are positioned in the magneticfield between the magnet 41 (FIG. 14) and the first wall portion 45 a ofthe stationary yoke 45. The interaction between the current in the sides61 and the magnetic field generates the electromagnetic force in thedirection of rotating the lens holder 1 about the support shaft 3.

In order to correct the focusing error, current is applied to thefocusing coil 42. The interaction between the current and the magneticfield caused by the magnet 41 generates the electromagnetic force formoving the lens holder 1 along the support shaft 3. By controlling theposition of the lens holder 1 in the direction along the support shaft3, the distance between the objective lens 2 and the recording media isadjusted, so that the focusing error is corrected.

The magnet 41 faces a cut-away portion 45 d formed in the first wallportion 45 a of the stationary yoke 45. The movement of the lens holder1 causes a change in the magnetic field between the magnet 41 and thefirst wall portion 45 a of the stationary yoke 45, so that a magneticrestoring force is generated according to the displacement of the lensholder 1. The shape of the cut-away portion 45 d is determined in such amanner that the linear property is obtained in the range (generally,approximately within ±0.5 mm) of the correction of the focusing error ofthe objective lens 2. Further, the length of the cut-away portion 45 din the direction of the optical axis of the objective lens 2 needs to begreater than the range of movement of the lens holder 1 for correctingthe focusing error.

In order to correct the tracking error, current is applied to thetracking coils 5 a and 5 b. The interaction between the current and themagnetic field caused by the magnet 41 generates the electromagneticforce for rotating the lens holder 1 about the support shaft 3. Thetracking error is corrected by controlling the position of the lensholder 1 in the direction traversing the track line of the recordingmedia.

The rotation of the lens holder 1 causes a change in the magnetic fieldbetween the magnet 41 and the first wall portion 45 a of the stationaryyoke 45, so that a magnetic restoring force is generated according tothe amount of rotation of the lens holder 1. The shape of the cut-awayportion 45 d is determined in such a manner that the linear property isobtained in the range (generally, approximately within ±0.5 mm) of thecorrection of the tracking error of the objective lens 2.

In addition to the restoring force described above, an attractivemagnetic force is exerted between the first wall portion 45 a of thestationary yoke 45 and the magnet 41. As a result, the bearing portion 1e of the lens holder 1 abuts against the support shaft 3, so that thebacklash of the lens holder 1 resulting from the gap between the bearingportion 1 e and the support shaft 3 is restricted.

As constructed above, according to Embodiment 4, in the objective lensdriving apparatus having the movable magnet, the manufacturing cost ofthe lens driving apparatus can be reduced, and therefore themanufacturing process can be simplified.

In Embodiment 4, the stationary base 9 and the stationary yoke 45 areseparately provided. However, it is possible that the stationary base ismade to have a function as the stationary yoke. In other word, thestationary base and the stationary yoke can be unitarily formed.

While the preferred embodiments of the present invention have beenillustrated in detail, it should be apparent that modifications andimprovements may be made to the invention without departing from thespirit and scope of the invention as described in the following claims.

1. An objective lens driving apparatus comprising: a lens holder as amovable part, which holds an objective lens; a stationary part thatsupports said lens holder in such a manner that said lens holder ismovable in a direction of an optical axis of said objective lens androtatable about a rotation axis parallel to said optical axis; a magnetmounted to said stationary part, said magnet being polarized in adirection toward or away from said rotation axis; a focusing coilmounted to said lens holder, said focusing coil having a sidesubstantially perpendicular to said optical axis and to said directionof polarization of said magnet; a tracking coil mounted to said lensholder, said tracking coil having a side substantially parallel to saidoptical axis, a magnetic path forming member that forms a magnetic pathin which a magnetic field caused by said magnet passes through said sideof said focusing coil and said side of said tracking coil; a currentsupplying member that supplies current to said focusing coil and saidtracking coil; and an urging member causing a magnetic force incombination with said magnet, for urging said lens holder toward areference position in a direction of said optical axis and in adirection of the rotation of said lens holder, wherein said urgingmember is made of a magnetic material and has a shape surrounding saidrotation axis, and the dimension of said urging member in the directionof said optical axis is smaller than that of said magnet.
 2. Theobjective lens driving apparatus according to claim 1, wherein saidurging member has at least one projection projecting substantially inparallel to the direction of the polarization of said magnet.
 3. Theobjective lens driving apparatus according to claim 1, wherein saidurging member has an asymmetric shape about said rotation axis in aplane perpendicular to said rotation axis.
 4. The objective lens drivingapparatus according to claim 2, said side of said tracking coilsubstantially faces the center of said magnet in the direction of therotation of said lens holder, and said projection of said urging membersubstantially faces the end of said magnet in the direction of therotation of said lens holder.
 5. The objective lens driving apparatusaccording to claim 1, wherein the positions of said urging member andsaid focusing coil are different from each other in the direction ofsaid optical axis.